maxTemperature.js

    var temperatureHistory = [{ 
      State: 'Florida',
      Temperature: 83,
    },
    { 
      State: 'Florida',
      Temperature: 82,
    },
    { 
      State: 'Florida',
      Temperature: 79,
    },
    { 
      State: 'Nevada',
      Temperature: 96,
    },
    { 
      State: 'Nevada',
      Temperature: 93,
    }];


    var max = temperatureHistory.reduce(function (topTemp, history) {
       if (history.Temperature > topTemp[history.State] || -99999) {
         topTemp[history.State] = history.Temperature;
       }
       return topTemp;
    }, {});

    console.log(max);